Prepare 2.8.1 release

[pganssle]

Summary of changes

This updates the changelog for the 2.8.1 release, which includes the following changes:

Version 2.8.1 (2019-11-03)

Data updates

• Updated tzdata version to 2019c.

Bugfixes

• Fixed a race condition in the tzoffset and tzstr "strong"
  caches on Python 2.7. Reported by @kainjow (gh issue Intermittent parse() KeyError failure with 2.8.0 #901).
• Parsing errors will now raise ParserError, a subclass of
  ValueError, which has a nicer string representation. Patch by
  @gfyoung (gh pr Use custom ParserError class in parser exceptions #881).
• parser.parse will now raise TypeError when tzinfos is passed a
  type that cannot be interpreted as a time zone. Prior to this
  change, it would raise an UnboundLocalError instead. Patch by
  @jbrockmendel (gh pr Add+collect tests for tzinfos input types, fix missed case of invalid… #891).
• Changed error message raised when when passing a bytes object as
  the time zone name to gettz in Python 3. Reported and fixed by
  @labrys () (gh issue gettz on Python 3 fails with TypeError when passed bytes #927, gh pr Fix error message for gettz when passed bytes #935).
• Changed compatibility logic to support a potential Python 4.0
  release. Patch by Hugo van Kemenade (gh pr Fix for Python 4 #950).
• Updated many modules to use tz.UTC in favor of tz.tzutc()
  internally, to avoid an unnecessary function call. (gh pr Use tz.UTC in favor of tz.tzutc() where possible. #910).
• Fixed issue where dateutil.tz was using a backported version of
  contextlib.nullcontext even in Python 3.7 due to a malformed
  import statement. (gh pr Fix incorrect import in Python 3.7+ #963).

Tests

• Switched from using assertWarns to using pytest.warns in the test
  suite. (gh pr Remove WarningTestMixin in favor of pytest.warns #969).
• Fix typo in setup.cfg causing PendingDeprecationWarning to not be
  explicitly specified as an error in the warnings filter. (gh pr
  Fix typo in setup.cfg warning configuration #966)
• Fixed issue where test_tzlocal_offset_equal would fail in certain
  environments (such as FreeBSD) due to an invalid assumption about
  what time zone names are provided. Reported and fixed by Kubilay
  Kocak (gh issue Test failure: test_tzlocal_offset_equal[GMT-tzoff1] (dateutil 2.8.0) #918, pr Fix test_tzlocal_offset_equal[GMT-tzoff1] #928).
• Fixed a minor bug in test_isoparser related to bytes/str
  handling. Fixed by @fhuang5 (gh issue bytes/str conversion lines not hit in test_isoparser #776, gh pr Improve coverage in test_isoparser #879).
• Explicitly listed all markers used in the pytest configuration. (gh
  pr Register pytest markers #915)
• Extensive improvements to the parser test suite, including the
  adoption of pytest-style tests and the addition of parametrization
  of several test cases. Patches by @jbrockmendel (gh prs use setup_class instead of setUp #735,
  Remove duplicate tests #890, Add several parser test cases #892, Parametrize tests for ignore_tz, dayfirst, yearfirst #894).
• Added tests for tzinfos input types. Patch by @jbrockmendel (gh pr
  Add+collect tests for tzinfos input types, fix missed case of invalid… #891).
• Fixed failure of test suite when changing the TZ variable is
  forbidden. Patch by @shadchin (gh pr Fix tests if not TZ change allowed #893).
• Pinned all test dependencies on Python 3.3. (gh prs Pin requirements more thoroughly on py33 #934, Pin colorama on Windows #962)

Documentation changes

• Fixed many misspellings, typos and styling errors in the comments
  and documentation. Patch by Hugo van Kemenade (gh pr Fix typos #952).

Misc

• Added Python 3.8 to the trove classifiers. (gh pr Add Python 3.8 to trove classifiers and Travis. #970)
• Moved as many keys from setup.py to setup.cfg as possible. Fixed
  by @FakeNameSE, @aquinlan82, @jachen20, and @gurgenz221 (gh
  issue Move all simple constants from setup.py into setup.cfg #871, gh pr Convert setup.py to declarative build file when possible #880).
• Reorganized parser methods by functionality. Patch by
  @jbrockmendel (gh pr REF: collect parser methods by function #882).
• Switched release.py over to using pep517.build for creating
  releases, rather than direct invocations of setup.py. Fixed by
  @smeng10 (gh issue Use pep517.build in release.py #869, gh pr add fix for 2019hackIllinois Use pep517.build in release.py #869 #875).
• Added a "build" environment into the tox configuration, to handle
  dependency management when making releases. Fixed by @smeng10 (gh
  issue Add a "build" job in tox #870,r gh pr Fix the add a build job in tox #870 #876).
• GH Don't upload coverage on azure #916, GH Updates to the release process #971

If you are listed here and would like to make any changes, unfortunately I'm springing this on you quite late in the game, as I plan to make a release tonight some time between 2019-11-03T05:00Z and 2019-11-03T06:59Z because I find it funny to make a release during an ambiguous datetime in my current local time zone (America/New_York).

[jacobg]

This rare parse() KeyError occurred for me running dateutil 2.8.1 on Python 2.7 (Google App Engine). It doesn't seem like this bug is fixed.